(1) Field of the Invention
This invention relates to protection of output devices, particularly to the output device of a voltage regulator, and output driver amplifier.
(2) Brief Description of the Related Art
Large output devices are commonly found in voltage regulator, output driver amplifier etc. Large surge current are produced especially during power supply powering up, as well as by power up/down control bit.
In a widely used regulated power supply as shown in FIG. 1, this surge current is particularly severe. In this operation, an unregulated supply voltage Vsup is applied through a p-channel MOS pass transistor M1A to a load Rload in parallel with a load capacitor Cload with a regulated output voltage Vout. The output voltage or a fraction of the output voltage is compared with a reference voltage Vref in a differential amplifier AMP. The output voltage of the differential amplifier Vg is used to control the gate of the pass transistor M1A until the regulated output voltage Vout is equal to the reference Vref. For proper operation, the output voltage Vout is applied to the inverting input of the differential amplifier AMP and the reference voltage Vref is applied to the non-inverting input of the differential amplifier AMP.
During the time when the power supply is suddenly applied (ramps up), the reference voltages appears at the non-inverting input of AMP before Vout appears at the inverting input of AMP due to the load capacitor. Thus, the gate voltage Vg of M1A is pulled down to cause a heavy current to flow in M1A. Such a surge current may damage the transistor.
The first prior art to reduce the surge current is to use diodes to clamp the gate voltage (FIG. 1, node Vg). The two pMOS transistors Md1 and Md2 are connected as diodes to clamp Vg to approxinately two threshold voltages below the supply voltage Vsup. However, it is difficult to obtain effective diode clamp that has the right trigger voltage and low leakage current during off state.
The second prior art is to control the node Vg change slowly during transient events as shown in FIG. 1. The Delay block generates a very slow delay ramp signal to slowly turn on the gate node Vg of the M1A, so as to try to reduce the large transient current. The result is not satisfactory due to the large output device current produced in response to small voltage change in Vg. Also, a too slow or weak control of the Vg conflicts with the control by the AMP amplifier.